Fifteen thousandths doesn't sound like a whole lot of free-bore. If I remember correctly, Weatherby rifles have much more than that, right from the factory. How does the crown on the muzzle look? A poorly machined or damaged crown can ruin accuracy in a hurry. The good news is that recrowning the barrel should be inexpensive and easy to do.

After you've exhausted all other possible solutions (ammo, bullet seating depth, scope, crown, etc.), and if your current gunsmith is unable/unwilling to help you, another gunsmith should be able to turn down the back of the barrel and recut the chamber. In effect, you can start over, without having to buy a new barrel or spend another $700.

Good luck.